Why could mice with early Alzheimer's disease not recall a fear they had learned? · Grey Matter

In the 2016 study the mice learned normally and failed only days later, and light on their tagged engram cells brought the fear back, so the memory was stored and the retrieval was broken.

Why could mice with early Alzheimer's disease not recall a fear they had learned?

In the 2016 study the mice learned normally and failed only days later, and light on their tagged engram cells brought the fear back, so the memory was stored and the retrieval was broken. The engram cells in the dentate gyrus had fewer dendritic spines, the small protrusions where synapses sit, and the loss grew with age before any amyloid plaques appeared. A single burst of light restored recall for about a day; repeated high-frequency stimulation regrew the spines to normal numbers and recall lasted up to six days. The fault lay in the connections that lead back to the trace.